The present invention relates to apparatus for removing volatile substances or the like from viscous materials such as polymer substance. This process is termed devolatilization. A polymer substance which is to be devolatilized is fed into a cylindrical cage and carried through the cage by screw members. The polymer is moved through a vented section of the cage where the pressure on the polymer is substantially reduced, permitting the volatile substance to vaporize and escape. The devolatilized polymer is then carried into an enclosed screw section for further working or for extrusion sizing.
One such devolatilizing device is shown in U.S. Pat. No. 3,684,252, to Nissle et al, issued Aug. 15, 1972, and assigned to the assignee of the present invention. Nissle et al shows devolatilizing apparatus in which the vent section includes a number of slit-like vent openings around the periphery of the devolatilizer cage. As the polymer material passes through the vent section of the cages the resulting vapors pass through the slit-like vent openings. The vapor is then pumped to a recovery apparatus.
In such an arrangement it is very difficult to keep the slit-like vent openings from becoming clogged. As the volatile substance evaporates in a vent section, small pieces of polymer, called "fines", may be carried by the vapor into the slit-like openings, obstructing these openings and inhibiting the venting process. Since the vent section must be totally enclosed to recover the vapors, the accumulated fines and extrusions may be cleaned only by a complete shut-down and vent disassembly.
The problem of keeping a vent in such a device free of obstructions has been recognized in the past. U.S. Pat. No. 3,350,742, issued Nov. 7, 1967 to Wood shows a vent including a vent liner which is to be replaced periodically. A sight glass is provided in the top of the vent to permit inspection of the vent and liner. Although reducing the down time of the devolatilizer, this vent liner arrangement still requires frequent maintenance and cleaning of the liners.
U.S. Pat. No. 3,737,150, issued June 5, 1973 to Otake shows a vent arrangement in which the vent tube extends through the wall of the cylindrical cage with the inner end of the tube terminating adjacent to the peripheral surface of the screw shaft. A notch in the end of the vent tube is intended to prevent the material being worked from moving up the vent tube and thus clogging it.
U.S. Pat. No. 3,212,133, issued Oct. 19, 1965 to Heidrich, shows an extruder having a vent duct which includes a screw conveyor for returning material to the cylindrical cage which is carried into the duct by the escaping volatile substances being vented. Such an arrangement requires a motor or actuator for the screw conveyor and further is difficult to clean and inspect.
U.S. Pat. No. 3,781,132, issued Dec. 25, 1973, to Latinen, discloses a devolatilizer having a vent which includes wedge-shaped members at the vent opening to prevent the polymer being worked from moving into the vent. This device is subject to clogging, however, since the wedges define a relatively small opening leading into the vent tube.
Extrusion devices in which the volatile matter vapor is not recycled, such as U.S. Pat. No. 3,834,440, issued to McCracken, on Sept. 10, 1974, are easier to vent. McCracken shows a device in which solvent vapor escapes from both the input and output openings of the extrusion dryer. While the input arrangement for the dryer includes a conical hopper, this hopper is continuously fed with the additional polymer material to be worked. The vented gases, therefore, must move upwardly through the material in the supply hopper and fines in the vapor are thus of little consequence.